Cleanroom environments are becoming increasingly required for certain industrial processes such as fabrication of semiconductor devices that are extremely sensitive to particle contamination. In some cases, submicron particles can produce defects hat may result in rejection or failure of the product. Measure normally used to minimize particle contamination in cleanrooms, in addition to scrupulous cleanliness, include filtration of room air, regulation of temperature and humidity, and requiring workers in the room to wear special outer garments that provide a barrier to the release of particles from clothing and the human body.
One of the best available fabrics for cleanroom garments is nonwoven, spunbonded polyolefin material manufactured and marketed by E. I. du Pont de Nemours and Company under the trademark Tyvek.sup..TM.. This fabric provides an effective barrier to transmission of particles from the wearer or his clothing, and it is amenable to being cleaned to an extent that its surfaces are virtually free of particles. Tyvek.sup.198 fabric, however, presents a problem in that cut edges or needle holes produced in stitching of seams may emit microscopic fiber particles in sufficient quantity to cause increased rejects. This source of contamination could be avoided by use of bonded, rather than stitched, seams in preparation of garments, with bonded seams that include folded-over edges being located on the inside of the garment. Forming bonded seams of this type has not proven feasible to date, however, owing to the fact that Tyvek.sup..TM. is not directly heat-bondable to another piece of the same material, this being indicated by its manufacturer When one attempts to weld the seam together in an ordinary technique such as by a continuous application of heat and pressure along the seam, the material undergoes a migration or change in composition, becoming hard and brittle.
Applicant's prior patent, U.S. Pat. No. 4,683,593, discloses the preparation of bonded seams of a composite material having a layer of Tyvek.sup..TM. and a layer of a film such as polyethylene, the seam being formed by continuous application of heat and intermittent application of pressure along the path of the seam. While effective for composite fabrics of the type described, this method, when applied to bonding Tyvek.sup..TM. to itself, produces a seam that has marginal strength and exposed cut edges on both sides of the fabric, thus reducing its effectiveness for cleanroom garment applications.